People sometimes use the terms cut and shape synonymously. But cut means more than shape and the Rapaport Price List has different price matrices for round and fancy shape diamonds. The Price List is actually for well cut diamonds. That has a more exact meaning in the case of round diamonds than it does for fancy shapes.
A really well cut round diamond has a table (top facet) that is between 55% and 59% of the stone’s diameter, a depth that is about 59-60%, star facets that are greater than 50% of the length from table to stone edge and lower girdle facets that are 80% or more of the length from the girdle to the culet. It will also have very good symmetry and polish meaning that the facets will be similarly shaped, not misshapen and be free of scratches, nicks, abrasions, etc. The Rapaport Price List is meant to be a starting point for negotiations between dealers for diamonds with actually somewhat more lenient standards than these.
While the cut of a diamond may be the most important factor in determining the optics of a diamond; how much brilliance, scintillation and dispersion the stone shows, it has the least effect on loan value. That is because there can be jumps or declines of hundreds of dollars per carat as you move from one clarity or color grade to another on the price list. And, since I am appraising most collateral in the mounting, it can be easy to be off by a grade on the color or clarity. The mounting limits your view, hides inclusions and masks color. So I rarely assign a premium to a well cut diamond since the Price List is for well cut diamonds anyway.
I have taken in as collateral many diamonds over the years that were cut in the 1950s, 60s and 70s that are deep and have large tables. These were cut at a time when manufacturers were more concerned about cutting the largest diamond from the rough rather than the best. If the optics are reasonably good, I usually do not downgrade the loan value because there will be buyers for the stone. If the optics are poor I will discount the loan value and I have even refused to accept as collateral stones that simply do not have the optical effects that we look for in diamonds and think of as beautiful.
Old European cut diamonds can be beautiful but they are an excuse the dealers use to offer 75% of what they would for a modern round brilliant cut diamond. And so their loan value reflects that market reality. Good cut in fancy shapes is somewhat harder to define but is affected by the length to width ratio of the stone, its depth and the attractiveness of the curves.